In general, resins such as polyethylene, polypropylene, polyester, polyurethane, polymethacrylate and epoxy resins have achieved wide use for many purposes including packages of electrical appliances, construction materials, fibers and daily commodities, because of their electrical insulating properties, strength, lightweight properties, heat resistance, water resistance and chemical resistance. However, since these general resins consist mainly of carbon, hydrogen and/or oxygen and thus readily catch fire, leading to combustion that evolves tremendous flames, heat and toxic smoke, their uses are extremely limited. This has led to development of various flame retardants. Known organic flame retardants include halogen flame retardants and phosphorous flame retardants. Known inorganic flame retardants include antimony oxide compounds, metal hydroxides such as magnesium oxide and aluminum hydroxide, and boron compounds. These are used alone or in combination. The selective combination of flame retardants such as, for example, a mixture of a halogen flame retardant and an antimony oxide compound, is known to have a synergistic effect. In Patent Literature 1, for example, a resin composition is disclosed which contains a halogen flame retardant and an antimony oxide compound. However, this resin composition evolves a toxic halogen (bromine and chlorine) gas when it is exposed to high temperature or processed, which has been a problem. Also, the antimony oxide compound is toxic and possible to cause antimony poisoning. Therefore, its use is undesirable for reasons of environment and health.
In Patent Literature 2, a phosphorous flame retardant is disclosed. However, if a sufficient flame-retardant effect is to be obtained with the sole use of this flame retardant, it must be incorporated in the amount of at least 10% by weight, based on 100 parts by weight of the resin. This causes hydrolysis of the resin at high temperature and accordingly lowers its physical properties.
Also in case of magnesium hydroxide or aluminum hydroxide, it must be incorporated in a large amount in order to impart sufficient flame retardance. This lowers mechanical strength.
Use of an inorganic layered compound (silicate such as montmorillonite) as a flame retardant has also been proposed. In Patent Literature 3, a layered clay mineral/halogen flame retardant/antimony oxide combination is proposed. In Patent Literature 4, a layered clay mineral/phosphate ester flame retardant/antimony oxide combination is proposed. However, these use a toxic halogen flame retardant or antimony oxide compound and are thus undesirable for reasons of environment and health.
After all, a flame retardant for a general resin has not been developed so far which can avoid environmental and health concerns and achieve a UL94 flame retardance rating of V-O or V-1 without impairing intrinsic properties of the resin.    Patent Literature 1: Japanese Patent Laid-Open No. Hei 8-109287    Patent Literature 2: Japanese Patent Laid-Open No. Hei 11-106597    Patent Literature 3: Japanese Patent Laid-Open No. Hei 11-189710    Patent Literature 4: Japanese Patent Laid-Open No. Hei 11-228748    Patent Literature 5: Japanese Patent Registration No. 2979132    Patent Literature 6: International Publication No. 99/11574 pamphlet    Patent Literature 7: Japanese Patent Registration No. 3062497    Patent Literature 8: International Publication No. 03/016218 pamphlet    Patent Literature 9: Japanese Patent Laid-Open No. Hei 6-93133